The ongoing miniaturization in integrated circuits with increased complexity and multilevel metal layers and the focus on increasing speed of these circuits has increased the demand for low permittivity materials, particularly for use as intermetal dielectric layers. Conventionally, metal interconnects, mostly aluminum layers, with silicon dioxide as intermetal dielectric are used, but this conventional solution will not be able to meet the stringent specifications resulting from the above mentioned trends. Therefore, to avoid that the larger portion of the total circuit delay is caused by the resistance and capacitance of the interconnect system, the permittivity of the dielectric used has to be reduced. This is stated in numerous publications, e.g. in Table 1 of R. K. Laxman, "Low .di-elect cons. dielectric: CVD Fluorinated Silicon Dioxides", Semiconductor International, May 1995, pp. 71-74. Therefore miniaturization has lead to an intensified search for new low K materials. A low .di-elect cons. material, a low K material and a material with a low permittivity are all alternative expressions for a material with a low dielectric constant, at least for the purposes of this disclosure.
Part of the search for new low K materials has been directed to changing the properties of silicon dioxide as deposited. Besides the focus has been on changing the properties of silicon oxide, there is an ongoing search for new low K materials. Among these new materials are the organic spin-on materials, having a K value in the range from 2.5 to 3, and the inorganic low-K materials as e.g. xerogels having a K value typically lower than 1.5. An important characteristic of these new materials is their porosity, i.e. the volumes of the pores as well as the pore size distribution. The relative pore volume directly defines the permittivity value and can be estimated by measurement of the dielectric constant using spectroscopic ellipsometry and porosity/density simulation as e.g. in T. Ramos et al., "Low-Dielectric Constant Materials", Mater. Res. Soc. Proc. 443, Pittsburgh, Pa. 1997, p.91. However, it is much more difficult to measure the pore size distribution. The pore size distribution defines mechanical, thermal and chemical properties of the porous materials. Therefore, by knowing the pore size distribution, one has a clear indication of the compatibility of the material with the manufacturing process of integrated circuits. If the pores are open, information about the pore size distribution can be obtained by adsorption porometry.
Adsorption porometry is based on the well-known phenomenon of hysteresis loop that appears in the processes of capillary condensation and desorption of vapour out of porous adsorbents. The theory of capillary condensation, as in S. J. Gregg and S. W. Sing, "Adsorption, Surface Area and Porosity", Acad. Pr., NY, 1982, explains the appearance of hysteresis by the change in the equilibrium vapour pressure above the concave meniscus of the liquid. Vapour can condense in the pores of a solid substrate even if its relative pressure is below unitary value, i.e. there is condensation even when the vapour pressure is less than the atmospheric pressure. Dependence of the relative pressure on the meniscus curvature is described by Kelvin equation: EQU 1n(P/P.sub.0)=-2.gamma.V.sub.L /(r.sub.m RT),
where P/P.sub.0 is the relative pressure of the vapour in equilibrium with the liquid, the surface of the liquid being a meniscus with the curvature radius r.sub.m ; .gamma. and V.sub.L are the surface tension and molar volume of the liquid adsorbate, respectively. The curvature radius r.sub.m is close to the pore radius. Adsorption-desorption hysteresis appears if the radius of curvature of the meniscus of the condensing liquid is changed as a result of adsorption. Every P/P.sub.0 value corresponds to a definite r.sub.m. Only spheroidal menisci are formed during desorption, while adsorption results in either spheroidal or cylindrical menisci. Because of this, it is more convenient to use desorption isotherms to determine the effective size of pores equivalent to cylindrical ones.
A method of wide application is adsorption porometry with the use of liquid nitrogen as in S. J. Gregg et al., "Adsorption, Surface Area and Porosity", Acad. Pr., NY, 1982. This state-of-the-art method is however only applicable when analyzing large samples because this method is based on direct weighing of the samples during the vapour adsorption and desorption. Therefore, this destructive method is inappropriate for analyzing thin films formed on a substrate. In some cases, in order to characterize the pore size distribution using this state-of-the-art method, it is necessary to damage the films of several tens of substrates. Moreover, the very low temperature which is required for nitrogen porometry also creates additional problems.